Controlling or diverting the flow of relatively viscous flowable materials presents problems not typically encountered when dealing with more flowable liquids. Typical examples of such viscous materials include various polymer resins and high-viscosity fibrous flowable materials, having viscosities typically in the range of 30,000 to 300,000 centipoise. The interior surfaces of valves and other flow elements must be especially smooth to reduce frictional drag at the boundary layer of the flowing material, and the valves should lack any dead spaces where the flowable material could collect and harden, and degrade.
These problems and considerations are particularly relevant in flow control valves, which divert the flow of viscous material from a normal process stream to an alternative flow path. This flow diversion may take place, for example, in polymer systems where the system is periodically cleaned by injecting a solvent upstream of various process elements, to clean the valves and conduits and remove accretions of the viscous material therein. When a solvent is injected in a flow line upstream of a process stage, the normal flow must be diverted to avoid contaminating the downstream process with the solvent or solvent-polymer mixture. It is particularly important that the wetted surfaces of such control valves be smooth and devoid of any dead spaces where polymer or other viscous material buildup can occur, regardless of valve position. Because the wetted surfaces within the valve must be polished to attain the desired level of smoothness, conventional valve design and fabrication has been less than satisfactory.